Mg-Zn-Y Magnesium Alloys Corrosion Fatigue Behavior

1. Why Magnesium for Lightweighting?

Magnesium is the lightest structural metal in engineering use, with a density of approximately 1.74 g/cm³ — roughly two-thirds that of aluminum (2.70 g/cm³) and one-quarter that of steel. Its specific strength (strength-to-density ratio) is competitive with aluminum alloys, and it offers good dimensional stability, excellent electromagnetic shielding, high damping capacity, and full recyclability.

These properties make magnesium alloys attractive for automotive and aerospace mass reduction programs, where even modest density reductions translate to fuel efficiency and payload gains. The automotive industry has used cast magnesium (AZ91, AM60) in instrument panels, gearbox housings, and seat frames for decades. Aerospace applications — including helicopter gearboxes and airframe brackets — have employed wrought alloys such as WE43 and ZK60.

The remaining obstacle to broader deployment is fatigue performance, particularly in corrosive environments.

2. The Fundamental Fatigue Challenge in Magnesium Alloys

HCP Crystal Structure and Limited Slip Systems

Magnesium crystallizes in the hexagonal close-packed (HCP) structure. Unlike face-centered cubic (FCC) metals such as aluminum, which have 12 readily activated slip systems at room temperature, magnesium HCP at ambient temperature activates primarily only the basal slip system {0001}⟨11¯20⟩. Non-basal slip systems (prismatic, pyramidal) require significantly higher critical resolved shear stress and are relatively inactive below ~200°C.

This restricted plasticity has two direct consequences for fatigue:

• Deformation twinning: When basal slip is insufficient to accommodate imposed strain, deformation proceeds by twinning — particularly extension twinning {10¯12} under tension and contraction twinning {10¯11} under compression. Twinning is directional (it operates differently under tension and compression), which produces a pronounced tension–compression yield asymmetry. This asymmetry generates non-symmetric hysteresis loops under fully-reversed fatigue loading that are not observed in FCC metals.
• Fatigue crack initiation at twins: Twin boundaries are preferred sites for fatigue crack initiation, particularly contraction twin boundaries under compressive half-cycles. The local stress concentration at twin-matrix interfaces accelerates microcrack nucleation.

Texture and Anisotropy

Thermomechanical processing — extrusion, rolling, forging — aligns HCP grains into strong crystallographic textures. Extruded magnesium alloys typically develop a fiber texture in which basal planes are preferentially parallel to the extrusion direction. This produces:

• High tensile yield strength along the extrusion direction (basal planes unfavorably oriented for slip)
• Low compressive yield strength (extension twinning easily activated)
• Tension–compression yield strength ratio (TYS/CYS) as low as 0.5–0.7 in some alloys
• Strong directional fatigue behavior — specimens loaded transverse to the extrusion axis can have significantly different S-N behavior than longitudinal specimens

Corrosion Susceptibility

Magnesium is electrochemically active — its standard electrode potential is −2.37 V vs. SHE, the most negative of all common structural metals. In chloride-containing environments (salt spray, seawater, humid marine air), magnesium corrodes via:

• General dissolution of the Mg matrix
• Pitting corrosion at second-phase particles and grain boundaries where galvanic couples form
• Stress corrosion cracking (SCC) under sustained tensile stress
• Corrosion fatigue — synergistic acceleration of fatigue crack initiation and growth by the corrosive environment

Corrosion pits act as stress concentrators with effective stress concentration factors K_t that can reach 2–4 depending on pit geometry, effectively eliminating the fatigue initiation phase and driving immediate crack propagation. This is why corrosion fatigue lives can be two to three orders of magnitude shorter than in-air fatigue lives for susceptible magnesium alloys.

3. Alloy Design Strategy: Mg–Zn–Y (ZW Series)

Standard commercial magnesium alloy families — Mg–Al–Zn (AZ series) and Mg–Al–Mn (AM series) — are well characterized for fatigue but limited in fatigue performance by coarse microstructures, brittle Mg₁₇Al₁₂ intermetallic phases, and relatively strong basal textures.

The Mg–Zn–Y alloy family addresses these limitations through two alloying strategies:

Zinc (Zn) — Solid Solution and Precipitation Strengthening

Zinc has moderate solid solubility in magnesium (~6.2 wt.% at the eutectic temperature, falling to ~2 wt.% at room temperature). It contributes solid solution strengthening and, at higher concentrations, precipitation of MgZn₂ and Mg₂Zn₁₁ phases. The ZW12 alloy contains only 1 wt.% Zn — a modest addition chosen primarily for its interaction with yttrium rather than for independent strengthening.

Yttrium (Y) — Texture Weakening and Recrystallization Control

Yttrium is not technically a rare-earth element (it falls in Group 3 of the periodic table but is chemically similar to the lanthanides and is often grouped with them). Its addition to magnesium has two well-documented effects critical to fatigue performance:

• Texture weakening: Yttrium segregates to grain boundaries and stacking faults during dynamic recrystallization (DRX) during hot extrusion. This segregation activates non-basal slip systems at lower temperatures, promoting more random grain orientations in the recrystallized material. The resulting weakened basal texture reduces the tension–compression yield asymmetry and produces more isotropic fatigue behavior.
• Recrystallization control: The Y content and extrusion parameters together determine whether the extruded microstructure is fully recrystallized (equiaxed grains, weak texture) or partially/non-recrystallized (retained deformation substructure, strong texture). This is the central variable in the Trujillo-Tadeo et al. study.

In ZW12 specifically — Mg–1Zn–2Y — the 2 wt.% Y addition is sufficient to significantly influence recrystallization behavior during extrusion without forming the large, brittle Mg₂₄Y₅ or W-phase (Mg₃Zn₃Y₂) intermetallics that appear at higher Y contents and which are detrimental to ductility and fatigue.

4. Experimental Program — Trujillo-Tadeo et al. (IFC14, 2026)

The study produced ZW12 extruded bars in three processing conditions by varying extrusion temperature, extrusion ratio, and ram speed:

Microstructural characterization used SEM/EDS for second-phase identification and EBSD for crystallographic texture quantification. Mechanical testing included uniaxial tension, compression, and high-cycle fatigue (HCF) at stress ratio R = 0 (tension–tension), run-out defined at 10⁷ cycles. Corrosion-fatigue specimens were pre-corroded in 0.5 wt.% NaCl solution then fatigue tested in air at the same stress levels.

5. Fatigue Results — In Air

The recrystallized conditions sustain run-out at 90% of yield strength — a remarkably high fatigue limit relative to yield strength. The non-recrystallized condition achieves run-out only at 70% YS, a 20 percentage-point penalty despite potentially having higher absolute yield strength due to the retained deformation substructure.

This result is mechanistically consistent: the non-recrystallized condition retains a strong basal texture and significant internal residual stress. Both features promote earlier fatigue crack initiation — strong texture concentrates slip activity on fewer favorably oriented slip planes, and residual tensile stresses add directly to the applied mean stress. The weakened basal texture in the recrystallized conditions distributes slip activity more uniformly, delays crack nucleation, and produces the superior fatigue ratio of ~0.90.

6. Fatigue Results — After Pre-Corrosion

Two of three conditions collapse under pre-corrosion — dropping from 10⁷-cycle run-outs to failures between 50,000 and 170,000 cycles. The 50 μm coarse-grained recrystallized condition sustains full run-out at 90% YS even after salt solution exposure.

Mechanistic Interpretation

The key driver is corrosion pitting. Pre-corrosion in 0.5 wt.% NaCl generates surface pits at susceptible microstructural sites — second-phase particles, grain boundaries, subgrain boundaries. These pits act as pre-existing stress concentrators that replace the fatigue crack initiation phase with immediate crack propagation, severely truncating total fatigue life.

The pit-to-crack transition is governed by the pit stress intensity factor:

When K_pit exceeds the fatigue crack propagation threshold ΔK_th, the pit transitions directly to a propagating fatigue crack. The coarse-grained (50 μm) recrystallized condition’s corrosion tolerance is attributed to two factors:

1. More uniform corrosion attack: Fewer grain boundaries per unit area reduce the density of preferential corrosion sites. Pits are more widely spaced and less likely to coalesce into larger defects.
2. Weak basal texture: More isotropic slip behavior distributes plastic strain at the pit tip over more grains, slowing the pit-to-crack transition rate.

The finer-grained (25 μm) recrystallized condition, despite also having weakened texture, is more susceptible because higher grain boundary density provides more pitting nucleation sites. The non-recrystallized condition fails for a different reason: its strong basal texture and subgrain boundaries both serve as pitting initiation sites and provide highly localized slip paths from pit tips.

7. S-N Curve Implications

Magnesium alloys do not exhibit a true fatigue endurance limit in the manner of ferrous alloys. The Basquin S-N relationship for the finite-life regime is:

where σ’_f is the fatigue strength coefficient, N_f is cycles to failure, and b is the Basquin exponent (typically −0.05 to −0.15 for magnesium alloys). For corrosion-fatigue, the S-N curve loses its knee and exhibits a continuously declining slope — there is no stress amplitude below which the corrosive environment does not accelerate damage. The dramatic life collapse in two of three ZW12 conditions illustrates this: under corrosion, the S-N knee shifts from 10⁷ to below 10⁵ cycles.

8. Comparison to Common Magnesium Alloy Families

The ZW12 coarse-grained recrystallized condition’s fatigue ratio of ~0.90 — and its retention of that ratio after pre-corrosion — places it above all common commercial magnesium alloys in this comparison. Broader validation under continuous corrosion fatigue is still needed.

9. Process–Microstructure–Fatigue Linkage

The central engineering finding is that extrusion parameters control fatigue performance through a chain of causal links:

Fatigue life optimization for ZW12 is fundamentally a processing problem, not a composition problem. The practical prescription: target full recrystallization with coarser grain size (~50 μm) for components in corrosive environments.

10. Engineering Implications for Structural Applications

• Environmental service condition is primary: In-air S-N data are non-conservative for components exposed to humidity, salt spray, or aqueous media. The two-order-of-magnitude life difference demands explicit environmental specification in any fatigue life assessment.
• Mean stress effect (R = 0 vs. R = −1): Under fully reversed loading, compressive twinning is activated each cycle, typically reducing fatigue life relative to R = 0. A Haigh diagram specific to each processing condition is needed for general loading assessment.
• Miner’s Rule applicability: Sequence effects — particularly the ordering of high-stress cycles relative to corrosion pit formation — can produce non-linear damage accumulation. Linear damage summation may be non-conservative.
• Inspection intervals: Should be based on corrosion pit growth rate rather than fatigue crack growth rate. Pits must be detected before reaching the critical size for pit-to-crack transition.
• Surface treatments: Shot peening or micro-arc oxidation (MAO) coatings are expected to extend corrosion-fatigue life by introducing compressive residual stress and providing a corrosion barrier — demonstrated on AZ31 and ZK60, a natural next step for ZW12.

11. Summary

ZW12 demonstrates that Mg–Zn–Y alloys can achieve fatigue and corrosion-fatigue performance substantially superior to standard AZ-series and ZK-series magnesium alloys, provided that extrusion parameters are carefully controlled to produce a fully recrystallized, coarse-grained microstructure with weakened basal texture. The 50 μm recrystallized condition’s ability to sustain 10⁷-cycle run-out at 90% YS even after chloride pre-corrosion positions Mg–Zn–Y alloys as credible candidates for cyclically loaded lightweight structures in moderately corrosive environments.

References
Trujillo-Tadeo, J.J., Goitia, J., Victoria-Hernández, J., Letzig, D., Microstructure and Corrosion Effects on ZW12 Fatigue Performance, IFC14 — 14th International Fatigue Congress, 2026.
Helmholtz-Zentrum Hereon GmbH, Institute of Material and Process Design, Geesthacht, Germany.
ASTM E466, Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials.
ASTM G44, Standard Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5% Sodium Chloride Solution.
Mordike, B.L., Ebert, T., Magnesium: Properties — Applications — Potential, Materials Science and Engineering A, Vol. 302, 2001, pp. 37–45.
Wu, L. et al., Twinning–detwinning behavior during fatigue cycling of wrought Mg alloy AZ31B, Acta Materialia, Vol. 58, 2010, pp. 1309–1317.
Irvine, T., Stress Corrosion Cracking, VibrationData Blog, 2026.